I would say that he definitely is suffering from a Calcium Deficiency due to having no
UVB light at all. Without an adequate, strong
UVB light (a long tube, either a T8 or preferably a much stronger T5 UVB tube is necessary, not a compact or coil
UVB bulb, as they are far too week for any desert repties), Bearded Dragons cannot manufacture any Vitamin D3, cannot absorb any nutrition from the food they eat or the supplements they are given, including calcium and all other vitamins and minerals, and their bodies cannot use any of the nutrition they take in. This results in nutritional deficiencies, weak bone density, malnutrition, and MBD and possibly Vitamin B1 (Thiamine) deficiency disease. So yes, the lethargy, the lack of appetite, the trouble moving/walking, these are all symptoms resulting from MBD.
What
UVB light did you buy? What brand, model, wattage, and type (long tube, compact, or coil) is the UVB you got? Is it obstructed by anything, including a mesh lid to the tank? And how far away is it, in inches, from his main Basking Spot/Platform?
The only way to stop the MBD progression and possibly reverse some of the damage is to get him under an adequate, strong UVB tube that is mounted unobstructed and within the correct distance from his basking spot/platform, and having both this appropriate UVB tube and a bright white basking bulb on for at least 13-14 hours minimum every single day, and having all 3 temperature zones inside his tank within the correct temperature ranges. Also, he needs his live insects dusted in a Reptile-grade Calcium powder at least 5 times a week once he's under an adequate UVB tube (I'd do every day with Calcium in his case, at least until he's around 8 months old) and at least 3-4 days a week in a Reptile Multivitamin.
Can you post some photos of both him and his entire setup and lights?